Consider a situation where you use a Solace PubSub+ software event broker as your MQTT server, with applications red, blue, and green, each within its own Message VPN.
Say, for example, you plan to increase your deployed client count from 100,000 to 400,000 MQTT clients, all of which need to intercommunicate with QoS 1 subscriptions. A single software event broker—the largest of which supports 200,000 queues—is not sufficient at this scale. While this might be possible with an appliance, you might not be interested in maintaining hardware, and instead opt for three 200,000 software event brokers configured in a DMR network, as shown in the following diagram.
Here the applications have been spread across all three software event brokers, which are interconnected with internal DMR links, with a combined capacity of 600,000 queues. This configuration can easily handle the needed number of clients, and scale even higher by adding additional software event brokers to the network over time. No manual subscription management is needed, because DMR dynamically discovers the subscriptions needed by each broker.
You'll find instructions to set up a Horizontal Scaling example at Horizontal Scaling Configuration.